Coating compositions have long been used to produce coatings having desired coating characteristics. For instance, coating compositions have been used to enhance surface appearance, such as providing high gloss or low gloss. For certain applications in vehicle and other industrial applications it is necessary to prepare coatings which result in surfaces with reduced gloss and/or textured surfaces. For example, commercial vehicle bodies and signage parts should have matt surfaces to reduce glare and plastics parts, e.g., external trim parts on vehicles such as bumpers should receive dull and/or textured surfaces. In order to obtain the matt and/or textured surfaces, flatting agents or texture additives are added to the coating compounds. When the applied coating film dries, the flatting agent particles produce a micro-rough surface texture. As a result, the incident light is reflected in a diffuse manner and gives the observer the impression of a matt surface.
The flatting agent particles must be distributed homogeneously in the dry film. The flatting agents and/or texture additives may be incorporated as material in powder or paste form. Conventional flatting agent pastes generally contain, apart from the flatting agents, binders, solvents and optionally additives. By increasing the loading of the flatting agent in a coating composition, the gloss can be reduced. To attain low gloss topcoats, the matting pastes have to be loaded with an excess amount, typically more than 6 weight percent based on the total weight of the composition of the flatting agent in order to achieve reasonable coverage and a low gloss finish. Unfortunately, the flatting agent in these excessively loaded coating compositions tends to coagulate, especially if pyrogenic, untreated fumed silica is used. As a result, the conventional low gloss coatings resulting therefrom have visually unacceptable seedy appearance. Furthermore, the presence of the excessive amount of these flatting agents in a coating composition tends to make the resultant coatings more brittle than conventional coating compositions. Additionally, high loading of flattener may lead to high viscosity, which is difficult to apply. Thus, a need exists for durable low gloss coatings that do not have seedy appearance.
For ecological reasons, low-emission, high-solids thermoset systems are increasingly being used. Adequate flowability is a prerequisite for problem-free handling and good meterability of the high solid coatings. It is difficult to achieve good flowability with the above-mentioned high solids coatings containing flatting agents and/or texture additives because in order to obtain predetermined texture effects or degrees of gloss, the matting paste/coatings must contain such large proportions of texturing and/or flatting agents that the products are pasty and no longer free-flowing, making acceptable processing impossible. Others have attempted to avoid this problem by employing silicas that have minimal or low impact on viscosity, such as untreated or organically treated precipitated silicas or synthetic amorphous silicas. Others have tried to increase flatting, with minimal impact on viscosity, by using silica gels, polymer spheres, incompatible polymers or waxes. However, these type of flatteners are not efficient in flatting high solids thermoset coatings. Due to their high usage in the film, “clarity” of the low gloss coatings is reduced, leading to a “dirtier” or “muddier” appearance.
Furthermore, processability of the matting pastes using VOC exempt solvents like PCBTF, acetone, and/or t-butyl acetate can be difficult. Acetone has very low flash point and is thus hazardous for storage and processing. T-butyl acetate is fast evaporating and it is difficult to spray large surfaces when it is used in high levels. PCBTF is known to have poor solvency characteristics for many binders, especially CABs and certain lacquers. Therefore, VOC compliant Low Gloss Topcoats using matting pastes with superior appearance in terms of film clarity, excellent metal orientation, sag resistance, fast dry, excellent application over large parts, even gloss over the entire surface, gloss consistency under different conditions and film builds and durability has not been feasible.
Attempts have been made to produce low gloss coating using matting pastes with improved handling characteristics. For example, U.S. Pat. No. 6,686,412 teaches the use of thixotropic (meth)acrylic copolymers and/or thixotropic polyesters in the flatting agent paste. It is alleged that using such thixotropic polymers permits pastes to be filled with the required amounts of flatting and/or texturing agents and still result in free-flowing pastes.
Besides handling issues, preparations containing flatting and/or texturing agents generally have the problem that partially dried particles of said preparations or of the coating agents containing them fall from the edge of the coating container back into the liquid coating and may thus lead to pinholing. It is not possible to screen these particles from the liquid coating prior to application since the texturing agent would then be removed too.
Further, coatings containing flatting agents often do not result in a uniform spray pattern. Unsightly patches form, particularly on fairly large surfaces to be painted or coated. Moreover, preparations in paste form can be difficult to keep stable in storage and can exhibit settling and re-agglomeration of untreated, fumed silicas, especially after prolonged storage.
Accordingly, there exists a need to achieve coatings with a matting paste that have good storage stability and result in improved appearance, where there is no hazy sheen and the coating exhibits uniform gloss levels over larger surface areas, without a blotchy appearance.